The ether bond has recently been described as "the single most common and unifying structural feature which confers to both biological and xenobiotic compounds a high degree of resistance to biological mineralization" (White et al., 1996). This resistance occurs in ether-bonded compounds ranging from the complex natural product lignin, to simpler and widely used anthropogenic chemicals including several pesticides (Alexander, 1973), common solvents such as diethyl ether (DEE) (Alexander, 1973) and, more recently, gasoline additives such as methyl tert-butyl ether (MTBE) (Mormille et al., 1994; Yeh and Novak, 1994), tert-amyl methyl ether (TAME) and ethyl tert-butyl ether (ETBE). At present very little is known about the microbial degradation of simple alkyl ethers. For instance, there are only two reports of microorganisms utilizing DEE as a growth-supporting substrate but these reports did not investigate the possibility of growth-supporting contaminants (Heyden, 1974; Parales et al., 1994) (the importance of this issue has been recently illustrated by the observation that growth of Ancyclobacter aquaticus in the presence of 2-chloroethyl vinyl ether is supported by abiotic hydrolysis products of the ether rather than the ether compound itself (van den Wijngaard et al., 1993)).
Whereas DEE is used as an industrial solvent, MTBE is widely used in many modem gasoline formulations. MTBE acts as both an octane enhancer and as an oxygenating compound, thereby permitting both an elimination of alkyl-lead anti-knocking agents and a reduction in automobile carbon monoxide emissions. Current consumption of MTBE in the United States, the world's largest consumer, was recently estimated at approximately 2.0.times.10.sup.10 gallons/year (Ainsworth, 1991). There is currently considerable uncertainty about the long-term human health effects of MTBE exposure. The U.S. Environmental Protection Agency has issued a drinking-water advisory for MTBE of 20-40 .mu.g/l USEPA (1997) (Drinking Water Advisory: Consumer acceptability advice and health effects analysis on methyl tertiary-butyl ether (MtBE) Office of Water, EPA-822-F-97-009. USEPA, Washington, D.C.). Recently, MTBE has been detected in many urban groundwater supplies, most likely as the result of gasoline spills and leaking storage tanks (Squillace et al., 1996). Recent studies also indicate that MTBE is very poorly biodegradable in groundwater under a variety of redox conditions (Mormille et al., 1994; Yeh and Novak, 1994). MTBE degradation has been described for a mixed microbial culture in a pathway involving tert-butyl alcohol (TBA) (Salanitro et al., 1994). Three bacterial isolates have been reported to exhibit slow growth on MTBE and yeast extract (Mo et al., 1997) and the oxidation of MTBE by propane-oxidizing bacteria such as Mycobacterium vaccae has been described (Steffan et al., 1997). However, the affinity of these organisms for MTBE (or, more specifically, the K.sub.m of the MTBE degrading enzymes produced by these organisms) may not be sufficient to achieve the 20-40 .mu.g/l standard set by the EPA for MTBE levels in water.
It is an objective of the present invention to provide isolated microorganisms which degrade MTBE and/or chlorinated aliphatic hydrocarbons, and which have a high affinity for these compounds.
It is a further object of this invention to provide biofilters suitable for the bioremediation of MTBE.